treewide: add support for "lto" in PKG_BUILD_FLAGS

This reduces open coding and allows to easily add a knob to enable
it treewide, where chosen packages can still opt-out via "no-lto".

Some packages used LTO, but not the linker plugin. This unifies 'em
all to attempt to produce better code.
Quoting man gcc(1):
"This improves the quality of optimization by exposing more code to the
link-time optimizer."

Also use -flto=auto instead of -flto=jobserver, as it's not guaranteed
that every buildsystem uses +$(MAKE) correctly.

Signed-off-by: Andre Heider <a.heider@gmail.com>

package/system/mtd/Makefile

@@ -18,6 +18,7 @@ PKG_LICENSE:=GPL-2.0+
 PKG_LICENSE_FILES:=
 
 PKG_FLAGS:=nonshared
+PKG_BUILD_FLAGS:=lto
 
 include $(INCLUDE_DIR)/package.mk
 
@@ -36,8 +37,7 @@ endef
 target=$(firstword $(subst -, ,$(BOARD)))
 
 MAKE_FLAGS += TARGET="$(target)"
-TARGET_CFLAGS += -Dtarget_$(target)=1 -Wall -flto
-TARGET_LDFLAGS += -flto=jobserver
+TARGET_CFLAGS += -Dtarget_$(target)=1 -Wall
 
 ifdef CONFIG_MTD_REDBOOT_PARTS
   MAKE_FLAGS += FIS_SUPPORT=1

package/system/procd/Makefile

@@ -22,6 +22,7 @@ PKG_LICENSE_FILES:=
 
 PKG_MAINTAINER:=John Crispin <john@phrozen.org>
 
+PKG_BUILD_FLAGS:=lto
 PKG_ASLR_PIE_REGULAR:=1
 PKG_CONFIG_DEPENDS:= \
 	CONFIG_TARGET_INIT_PATH CONFIG_KERNEL_SECCOMP CONFIG_PROCD_SHOW_BOOT \
@@ -36,9 +37,6 @@ endif
 
 CMAKE_OPTIONS += -DEARLY_PATH="$(TARGET_INIT_PATH)"
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto
-
 define Package/procd/Default
   SECTION:=base
   CATEGORY:=Base system

package/system/ubus/Makefile

@@ -15,6 +15,7 @@ PKG_LICENSE:=LGPL-2.1
 PKG_LICENSE_FILES:=
 PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 
+PKG_BUILD_FLAGS:=lto
 PKG_ASLR_PIE_REGULAR:=1
 
 include $(INCLUDE_DIR)/package.mk
@@ -50,8 +51,7 @@ define Package/libubus-lua
   TITLE:=Lua binding for the OpenWrt RPC client
 endef
 
-TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include -flto
-TARGET_LDFLAGS += -flto
+TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include
 
 CMAKE_OPTIONS += \
 	-DLUAPATH=/usr/lib/lua \

package/system/usign/Makefile

@@ -10,7 +10,7 @@ PKG_SOURCE_VERSION:=f1f65026a94137c91b5466b149ef3ea3f20091e9
 PKG_MIRROR_HASH:=3f6569a5e63fdfd032976ac0f79d736d3935101ac1b97fb370514b013c5e6bb6
 CMAKE_INSTALL:=1
 PKG_CHECK_FORMAT_SECURITY:=1
-PKG_BUILD_FLAGS:=no-mips16
+PKG_BUILD_FLAGS:=no-mips16 lto
 
 PKG_LICENSE:=ISC
 PKG_LICENSE_FILES:=
@@ -30,9 +30,6 @@ define Package/usign
   TITLE:=OpenWrt signature verification utility
 endef
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto=jobserver
-
 CMAKE_OPTIONS += \
 	-DUSE_LIBUBOX=on